1. Field of the Invention
The present invention relates to a technology for calculating color balance of an image including a catch light area (highlight area where gradation has been lost).
2. Description of the Related Art
Recently, due to the spread of image-related apparatuses, such as digital cameras and color printers, and improvements in the performances of computers, consumers have had increased opportunities to process digital color images. However, qualities of color images obtained by the digital cameras do not always satisfy the users, namely, color cast images with lost color balances are not uncommon.
Various color balance correction techniques have been developed in the past for correcting the color cast. One of them detects catch light areas and excludes them to eliminate their effects, takes statistics for pixels, and performs correction based on the result of the statistics. For example, Japanese Patent Application Laid-open No. H9-238257 discloses a technique that excludes pixels with a brightness of a predetermined level or higher and a color of a predetermined level or lower as invalid pixels included in the catch light area, takes statistics for the remaining pixels, and corrects the color balance of the entire image based on the result of the statistics.
However, the conventional technique using the brightness and the color has a drawback that the catch light area cannot be identified accurately. FIG. 12 is an explanatory diagram of a catch light area that can not be identified with the conventional technique that uses the brightness and the color. An image shown in FIG. 12 is an image of a fluorescent lamp and two other objects irradiated with the fluorescent light of the fluorescent lamp. Due to the effect of the fluorescent light, the image is entirely occupied with yellow and the portion of the fluorescent lamp has almost collapsed.
The table below the image in FIG. 12 depicts, for each pixel, an RGB value and an L*a*b value. L* corresponds to brightness and a* and b* correspond to color. Among pixels of collapsed image, some of the pixels are not excluded as invalid pixels, because, it is determined in the conventional technique that these pixels have color because the value of b* is 30 or more. In other words, at least in the example shown in FIG. 12, the catch light area cannot always be accurately identified based only on the L*a*b value.